Multi-range vortex-type flowmeter

ABSTRACT

A flowmeter of the vortex type in which an obstacle assembly mounted in a flow tube causes vortices to be shed at a frequency that is a function of flow rate. The assembly includes a block having a predetermined geometry fixedly mounted across the flow tube at right angles to the direction of flow. Fitting over the front face of the block is a replaceable adapter cap having a channel therein which conforms to the profile of the block and acts as a socket therefor. The formation of the adapter cap is such as to effectively expand the cross-section of the block without altering the basic geometric configuration thereof whereby the resultant expanded block restricts the effective area in the flow tube of fluid passing by the block, thereby making it possible to measure flow rates below the normal operating range of the meter.

RELATED APPLICATION

This application is a continuation-in-part of my copending applicationSer. No. 768,416, filed Feb. 14, 1977 on a "Multi-Range Vortex-TypeFlowmeter" (now U.S. Pat. No. 4,062,238).

BACKGROUND OF INVENTION

This invention relates generally to flowmeters of the vortex type, andmore particularly to an adapter cap for the shedding body of a flowmeteracting to reduce the effective area of the fluid passing by the bodywithout altering the basic geometry of the body, thereby making itpossible to measure flow rates below the normal operating range of themeter.

It is well known that under certain circumstances the presence of anobstacle in a flow conduit will give rise to periodic vortices. Forsmall Reynolds numbers, the downstream wake is laminar in nature, but atincreasing Reynolds numbers, regular vortex patterns are formed. Thesepatterns are known as Karman vortex streets. The frequency at whichvortices are shed in a Karman vortex street is a function of flow rate,this phenomenon being exploited to create a flowmeter. Flowmeters of thevortex-shedding type are disclosed in U.S. Pat. Nos. 3,116,639 and No.3,572,117, among others.

U.S. Pat. No. 3,589,185 describes an improved form of vortex-typeflowmeter wherein the signal derived from the fluidic oscillation isrelatively strong and stable to afford a favorable signal-to-noiseratio, thereby insuring accurate flow-rate information over a broadrange. In this meter, an obstacle assembly is mounted in the flow tube,the assembly being constituted by a block positioned across the tubewith its longitudinal axis at right angles to the direction of fluidflow, a strip being similarly mounted across the conduit behind theblock and being spaced therefrom to define a gap which serves to trapKarman vortices and to strengthen and stabilize the vortex street. Thisstreet is sensed by a pressure or other form of transducer to produce anelectrical signal whose frequency is proportional to flow rate.

A conventional vortex-shedding flowmeter has a fixed metering rangewithin which it is capable of accurately measuring flow rate. This rangeis largely determined by linearity requirements, signal recoveryparameters and internal velocity limitations.

In some instances, it becomes necessary to accurately measure low flowvelocities which lie below the normal operating range of a standardvortex-type meter. With existing meters, it is not possible, in thefield, to alter the operating range of an installed meter. Moreover, itis difficult to produce a small capacity flowmeter; for with existingvortex-shedding meter structures, the vortex-sensing system cannoteconomically be miniaturized.

In U.S. Pat. No. 4,003,253 to Yard et al., there is disclosed an adapterfor a standard vortex-shedding flowmeter. This adapter, when applied tothe meter, renders it capable of measuring low flow velocities below thenormal operating range thereof, the adapter acting to restrict theeffective area of fluid traversing the vortex-shedding body, whereby forthe same velocity of flow past the shedding body, a smaller amount offluid is metered.

A significant feature of the Yard et al. invention is that by means ofadapters of different size, one can readily change the operating rangeof the meter in the field without, however, adversely affecting thelinearity of the meter.

In the Yard et al. arrangement, the adapter is constituted by a pair ofrods which are inserted longitudinally into the flow tube on oppositesides of the vortex-shedding block mounted transversely across the tube.These rods act to restrict the effective area of the fluid traversingthe block whereby for the same velocity of flow past the block, asmaller amount of fluid is metered.

The Yard et al. arrangement presents certain practical difficulties.Thus there is the problem of supporting the adapter rods within the flowtube so that they conform to the internal wall of the flow tube and yetcan be readily withdrawn from the tube. Second, there is the problemraised by the shape of the shedding body, for the inserted rods are onlyeffective with certain shedder shapes.

SUMMARY OF INVENTION

In view of the foregoing, the main object of this invention is toprovide an adapter for a vortex meter which renders the meter capable ofmeasuring low flow velocities below the normal operating range thereof.

More specifically, it is an object of this invention to provide anadapter of the above-noted type in the form of a cap which is receivableon a vortex shedding body disposed within a flow-tube, the cap servingto enlarge the effective cross-sectional area of the body withoutaltering its geometry, thereby restricting the flow passage through thetube and changing the capacity of the meter.

A significant feature of the invention is that by means of adapter capsof different size, one can readily change the operating range of themeter in the field without, however, adversely affecting the linearityof the meter.

Apart from lowering the meter capacity, the adapter cap offers anothermajor advantage; for the basic flowmeter can be designed with a narrowshedding body, resulting in better operation and a lower pressure dropat high fluid velocities, whereby when the cap is added, the meter isthen converted into a low fluid velocity measuring instrument.

This multi-range operating mode is especially attractive in largervortex type flowmeters; for when the cost of installing a line-sizedmeter is unduly high, it may be economically feasible to install asmaller meter capable of measuring the fluid at the higher metervelocities with an acceptable pressure drop. On the other hand, when aline-sized meter is installed, fluid velocities are usually relativelylow and a different set of optimum characteristics is required.

The use of cap adapters in accordance with the invention affords anadditional fringe benefit; for if the fluids being measured are highlyabrasive and tend to erode the surface of the obstacle body, caps can bechanged when necessary to recondition the flowmeter. Moreover, one mayprovide adapter caps whose abrasion resistance is greater than that ofthe obstacle body, thereby prolonging the life of the meter.

Briefly stated, these objects are attained in a flowmeter in which anobstacle assembly mounted in a flow tube includes a block having apredetermined geometry positioned across the tube at right angles to thedirection of flow, the front face of the block being presented to theincoming fluid whereby vortices are shed thereby to produce fluidicoscillations at a frequency proportional to flow rate.

Fitting over the face of the block is a removable adapter cap having achannel which conforms to the profile of the block and acts as a sockettherefor. The formation of the cap is such as to effectively expand thecross-sectional area of the block without altering the geometricconfiguration thereof, whereby the resultant expanded block restrictsthe effective area on the flow tube of fluid passing by the block,thereby making it possible to measure flow rates below the normaloperating range of the meter.

By the use of caps of different size which are mountable on the block toincrementally expand its effective crosssectional area, a multi-rangemeter may be provided. The geometry of the block may be such as to havea uniform cross-section throughout its length. Or the block may becontoured as to have a varying cross-section.

OUTLINE OF DRAWINGS

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following detaileddescription to be read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a longitudinal section taken through a vortex flowmeterincluding an obstacle assembly having an adapter cap in accordance witha first embodiment of the invention;

FIG. 2 is a section taken through the meter in the plane indicated byline 2--2 in FIG. 1;

FIG. 3 is a front view of the meter;

FIG. 4 separately illustrates, in perspective, the block included in theobstacle assembly on which the adapter cap is mounted;

FIG. 5 illustrates a second embodiment of an adapter cap for a block ofdifferent geometry;

FIG. 6 illustrates a third embodiment of an adapter cap for a block ofstill another shape;

FIG. 7 illustrates, in perspective, a fourth embodiment of an adaptercap for a contoured block;

FIG. 8 separately shows the contoured block; and

FIG. 9 separately shows the adapter cap for the contoured block.

DESCRIPTION OF INVENTION First Embodiment

In the vortex-type flowmeter of the type disclosed in the Herzl U.S.Pat. No. 3,867,839, there is provided an obstacle assembly adapted togenerate strong stabilized fluidic oscillations causing a deflectablesection of the assembly to vibrate at a corresponding rate. Thesemechanical vibrations are sensed to produce a signal whose frequency isproportional to the flow rate of the fluid. In the flowmeter disclosedin this patent, the mechanical vibrations are sensed by one or morestrain gauges mounted within the deflectable section to produce periodicchanges in electrical resistance, resulting in a signal whose frequencyis proportional to the vibratory rate and hence to the flow rate of thefluid.

The basic flowmeter illustrated in FIGS. 1 to 4 is of the type describedin greater detail in the Herzl and Stroheimer patent application Ser.No. 758,849, filed Jan. 12, 1977, entitled "Obstacle Assembly for VortexType Flowmeter," (now U.S. Pat. No. 4,052,895) in which the vibrationsof a deflectable section of the obstacle assembly are detected by aforce sensor which is external to the flow tube. It is to be understood,however, that while the invention is illustrated in an external sensorarrangement in order to show how the obstacle assembly provides adequatespace for a sensing system, in its broadest aspect, the invention, whichis primarily concerned with the geometry of the obstacle assembly, isapplicable to internal as well as external sensor arrangements forvortex-type flowmeters and to vortex meters with other sensingexpedients, such as thermistors, pressure sensors, etc.

FIGS. 1 to 4 illustrate a basic flowmeter having an obstacle assembly onwhich is mounted an adapter cap AC in accordance with one embodiment ofthe invention, the meter including a flow tube 10 interposed in thewater line for a waterflood system or in any other environment in whichit is necessary to conduct an occasional test of flow rate to determinewhether proper flow conditions exist. For this purpose, the flow tubemay be provided with end flanges to facilitate coupling to the line.

Mounted within flow tube 10 is an obstacle assembly generally designatedby numeral 11, the assembly including a deflectable section which isresponsive to the Karman vortex street and is caused to vibratemicroscopically at a frequency which is proportional to flow rate.Incorporated in the obstacle assembly is a vibration transmittercomposed of a rod 12 and a probe 13.

Flow tube 10, which is shown as having a circular cross-section,includes an inlet into which the fluid to be metered is introduced. Theflow impinges on obstacle assembly 11 which acts to divide the flowaround the obstacle, producing fluidic perturbations in the form of aKarman vortex street. Obstacle assembly 11 is constituted by atransversely-mounted front section 14 in the form of a block and a rearsection 15 mounted behind the front section by a cantilever support inthe form of a flexible beam 16.

Front section 14 is a block having a generally square cross-sectionwhich is uniform throughout the long axis of the block, this axis beingperpendicular to the flow axis X of the flow tube. The extremities ofthe front section are secured to the wall of the tube whereby the frontsection is held fixedly within the tube. Rear section 15 is constitutedby a rectangular vane which is maintained by beam 16 in spaced relationto the front section, the plane of the vane being parallel to the flatupper and lower faces of front section block 14.

Because rear section 15 is cantilevered by means of flexible beam 16, itis deflectable. The beam, though bendable, has sufficient rigidity so asto permit only a slight deflection of the rear section. As a consequenceof the fluidic oscillations produced within the flow tube, thedeflectable rear section 15 is excited into vibration at a ratecorresponding to the frequency of the oscillations.

The natural resonance of the deflectable rear section is such as to bewell outside the normal frequency range of the meter whereby mechanicalresonance peaks are absent and the amplitude of the vibrating motionaccurately reflects the amplitude of the fluidic oscillations. Thedownstream vane section of the assembly carries out two functions; forthis section which interferes with the wake not only stabilizes it toenhance its detectability, but its vibratory motion gives rise to theoutput signal.

Because the deflectable system is relatively rigid, the total excursionof the rear section is minute even at the highest amplitudes of fluidicoscillation, so that metal fatigue of the supporting beam, as a resultof the vibrating action, is minimized and failures do not arise afterprolonged operation.

It is important to note that the magnitude of deflection is not ofprimary importance, for the flow rate information is given by thefrequency, not the amplitude of vibration. Hence while the deflectionmagnitude is made extremely small in order to provide an acceptablefatigue life, this does not militate against a readable output ofvarying frequency.

The minute vibrations of the deflectable rear section of the obstacleassembly are sensed outside of flow tube 10 rather than within the tube.For this purpose, the vibrations are conveyed by the vibrationtransmitter, including rod 12, whose rear portion is socketed within abore 18 within beam 16, the bore extending to a point adjacentdeflectable section 15. The front portion of rod 12 lies freely within arelatively large diameter, longitudinally-extending bore 19communicating with the smaller diameter bore 18 and extending well intofront section 14.

Rod 12 terminates in a collar 12A which encircles the end portion ofprobe 13 to provide a link between the rod and the probe. Probe 13extends through a longitudinal passage in front section 14 and projectsthrough an opening in the wall of flow tube 10 which is covered by aflexible diaphragm 20, probe 13 terminating in a coupling head 21.

Any force sensor 22 capable of responding to a force developed atcoupling head 21 to produce a corresponding electrical signal may beused to provide a signal indicative of flow rate. A preferred sensor forthis purpose is a quartz piezoelectric load cell, such as the"Piezotron" load cell (922 series) manufactured by Kistler InstrumentCompany of Redmond, Wash. This is a very stiff, rugged force sensorresponsive to minute incremental forces and usable in environmentscontaminated by dust, dirt or moisture without any adverse effect onsignal transmission.

An obstacle body having a square cross-section is a mechanicallyefficient shape and provides sufficient internal volume to accommodatethe rod and probe for transmitting the vibrations to an external sensingpoint. But a square cross-sectional shape affords a poor sheddingaction. As noted in the above-identified copending Herzl and Stroheimerapplication, when the block is modified in the manner shown in FIGS. 1and 4, the shedding characteristics are significantly altered and thebody becomes a superb shedder. This shedder is particularly advantageousin small vortex-type flowmeters (1 to 8 inch internal diameter).

This modification will now be explained. The front section block havinga square cross-section is provided with a flat front face FF which has aheight A, a flat rear face RF and a flat upper and lower faces UF andLF, respectively.

Front face FF is presented to the incoming fluid stream, and rear faceRF is parallel thereto, the two faces being at right angles to thedirection of flow. The distance between front face FF and rear face RFis represented by symbol C, and this distance is about equal to heightA. The rear corners of block 14 are bevelled, the upper and lower bevelsb₁ and b₂ lying within a rear zone B' which is about 1/3 the distance C,so that the front zone B which encompasses the upper and lower flat faceis 2/3 distance C.

The angle of bevels b₁ and b₂ is not critical and may be in the range ofabout 45° to 60°. As a consequence, the area of rear face RF is smallcompared to the area of front face FF. The size of height A of the blockrelative to the internal diameter D of the flow tube is not critical andlies in the range of about 0.15 to 0.35. The distance E along beam 16between rear face RF of the block and the leading edge of vane 15 isabout 1 to 11/2 D, whereas the width F of vane 15 is about 1/4 thewavelength of the shedding frequency.

As indicated in the copending Herzl and Stroheimer application, with ashedding arrangement having the geometric relationships set forthhereinabove, meter linearities of better than ±1% can be achieved atReynolds numbers as low as 7000. Because the A to D relationship is notcritical, this allows much greater freedom in meter optimization.

Cap AC which is mounted over the front face FF of the block 14 has achannel therein which conforms to the geometry of the block and acts asa socket therefor. Thus the base C₁ of the cap matches front face FF ofthe block and the sides C₂ and C₃ match the upper and lower faces UF andLF of the block.

The formation of cap AC is such as to effectively expand thecross-sectional area of block 14 without altering its geometricconfiguration. Thus the front face C₄ of the cap is parallel to frontface FF of the block, the upper and lower faces UF and LF of the block,whereas the rear bevels C₇ and C₈ of the cap have the same angle and liein the same plane as rear bevels b₁ and b₂ of the block. By mounting thecap on the block, the geometric form and advantages of this sheddingbody are maintained, but the effective area of the flow passage betweenflow tube 10 and this body is reduced in size, making it possible forthe meter to measure flow rates below its normal operating range.

In practice, the cap may be secured to the block by screws or otherremovable means so that the cap may be replaced, when worn by abrasivefluids, by a fresh cap of the same size. Or the cap may be replaced byanother cap which also maintains the geometry of the block but whichprovides a different degree of fluid restriction to render the meteroperative in a different range.

Second Embodiment

The invention is by no means limited to adapter caps of a block whosegeometry is of the type shown in FIGS. 1 and 4 and is applicable toother obstacle block forms which lend themselves to expansion withoutchange in geometric configuration. Thus FIG. 5 shows a block 23 having arectangular cross-section whose cross-sectional area is expanded by anadapter cap AC¹ which maintains this geometric form but restricts theflow passage, to lower the range of the vortex meter containing thisobstacle.

Third Embodiment

FIG. 6 shows still another form of obstacle body 24 which has agenerally rectangular cross-section and is bevelled both at the frontand rear, the adapter cap AC¹¹ in this instance has a channel whichconforms to the shape of block 24 and enlarges its effectivecross-sectional area without altering its geometric configuration.

Fourth Embodiment

Referring now to FIG. 7, 8 and 9, there is shown a fourth embodiment ofan adapter cap AC³ which acts to expand the cross-section of a contouredblock 25. This shedding obstacle is more fully disclosed in aHerzl-Strohmeier copending application Ser. No. 818,733, filed July 25,1977, entitled "Contoured Obstacle Assembly for Vortex-Type Flowmeter,"the entire disclosure of which is incorporated herein by reference.

As explained in this copending application, the width of block 25 hasits maximum value at the midpoint thereof and diminishes graduallytoward either end where the width assumes its minimum value, such thatthe upper and lower faces of the block are bellied and the front facehas a double-convex contour. The maximum width value affords anefficient shedding action, whereas the mean width value which issubstantially smaller than the maximum value acts to significantlyreduce the pressure drop introduced by the meter in the flow line. Therear corners 25A and 25B of the block are bevelled.

Thus the contoured block has a generally rectangular cross-section whosearea varies throughout the length of the block. Adapter cap AC³ has achannel 26 which conforms to the profile of the block so that the blockis snugly received therein. Cap AC³ functions to enlarge the effectivecross-sectional area of block 25 throughout the length thereof withoutaltering the basic block geometry, thereby restricting the flow passageto lower the range of the flowmeter. Cap AC³ has rear bevels on eitherside of channel 26, the angles of these bevels corresponding to those ofthe rear corner bevels on block 25.

While there have been shown and described preferred embodiments of amulti-range vortex-type flowmeter in accordance with the invention, itwill be appreciated that many changes and modifications may be madetherein without, however, departing from the essential spirit thereof.

I claim:
 1. In combination with a flowmeter of the vortex type in whichan obstacle assembly mounted in a flow tube includes a block having apredetermined configuration fixedly mounted across the tube at rightangles to the direction of fluid flow, the front face of the block beingpresented to the incoming fluid, a replaceable adapter cap fitting overthe front face of the block, said cap having a longitudinal channeltherein which conforms to the profile of the block and acts as a sockettherefor, said cap having a formation effectively expanding thecross-sectional area of the block throughout the length of the blockwithout altering its geometric configuration whereby the resultantexpanded block restricts the effective area in the flow tube of thefluid passing by the block, thereby making it possible to measure flowrates below the normal operating range of the meter in the absence ofthe cap.
 2. In a combination as set forth in claim 1, wherein said blockhas a generally rectangular cross-section with bevelled rear corners,said cap having rear bevels on either side of said channel whose anglescorrespond to those of said block bevels.
 3. In a combination as setforth in claim 1, wherein said cap is made of a material which is morehighly resistant to abrasion than the material of said block.
 4. In acombination as set forth in claim 1, wherein said assembly furtherincludes a vane mounted behind said block and cantilevered therefrom bya flexible beam.
 5. In a combination as set forth in claim 1, whereinsaid block has a rectangular cross-sectional area and said adapter capenhances said rectangular cross-sectional area.
 6. In a combination asset forth in claim 1, wherein said block is contoured and has across-sectional area which varies throughout its length.